1. Field of the Invention
The present invention relates to electrochemical energy converters with polymer electrolyte membranes, such as fuel cells or electrolyzer cells or stacks of such cells. In particular, the present invention relates to systems and methods for assembling, compressing, isolating, and/or retaining stacks during and/or following fabrication.
2. Description of the Related Art
Electrochemical cells comprising polymer electrolyte membranes (“PEM”s) may be operated as fuel cells wherein a fuel and an oxidant are electrochemically converted at the cell electrodes to produce electrical power, or as electrolyzers wherein an external electrical current is passed between the cell electrodes, typically through water, resulting in generation of hydrogen and oxygen at the respective electrodes. FIGS. 1-4 collectively illustrate typical designs of a conventional membrane electrode assembly 5, an electrochemical cell 10 comprising a PEM layer 2, and a stack 100 of such cells.
Each cell 10 comprises a membrane electrode assembly (“MEA”) 5 such as that illustrated in an exploded view in FIG. 1. MEA 5 comprises an ion-conducting PEM layer 2 interposed between first and second electrode layers 1/3 which are typically porous and electrically conductive, and each of which comprises an electrocatalyst at its interface with the PEM layer for promoting the desired electrochemical reaction. The electrocatalyst generally defines the electrochemically active area of the cell. The MEA 5 is typically consolidated as a bonded, laminated assembly.
In an individual cell 10, illustrated in an exploded view in FIG. 2, an MEA 5 is interposed between first and second cell separator plates 11/12, which are typically fluid impermeable and electrically conductive. The cell separator plates 11/12 are are manufactured from non-metals, such as graphite; from metals, such as certain grades of steel or surface treated metals; or from electrically conductive plastic composite materials.
Fluid flow spaces, such as passages or chambers, are provided between the cell separator plates 11, 12 and the adjacent electrode layers 1, 3 to facilitate access of reactants to the electrode layers and removal of products. Such spaces may, for example, be provided by means of spacers between the separator plates 11, 12 and the corresponding electrode layers 1, 3, or by provision of a mesh or porous fluid flow layer between the cell separator plates and corresponding electrode layers. More commonly, channels are formed in the faces of the cell separator plates 11, 12 that face the electrode layers 1, 3. Cell separator plates 11, 12 comprising such channels are commonly referred to as fluid flow field plates. In conventional cells 10, resilient gaskets or seals are typically provided around the perimeter of the flow fields between the faces of the MEA 5 and each of the cell separator plates 11, 12 to prevent leakage of fluid reactant and product streams.
Electrochemical cells 10 with ion-conductive PEM layers 2, sometimes called PEM cells, are advantageously stacked to form a stack 100 (see FIG. 4) comprising a plurality of cells disposed between first and second end plates 17, 18. A compression mechanism is typically employed to hold the cells 10 tightly together, to maintain good electrical contact between components, and to compress the seals. In the embodiment illustrated in FIG. 3, each cell 10 comprises a pair of cell separator plates 11, 12 in a configuration with two cell separator plates per MEA 5. Cooling spaces or layers may be provided between some or all of the adjacent pairs of cell separator plates 11, 12 in the stack 100. An alternate configuration (not shown) has a single separator plate, or “bipolar plate,” interposed between a pair of MEAs 5 contacting the cathode of one cell and the anode of the adjacent cell, thus resulting in only one separator plate per MEA 5 in the stack 100 (except for the end cell). Such a stack 100 may comprise a cooling layer interposed between every few cells 10 of the stack, rather than between each adjacent pair of cells.
The illustrated cell elements have openings 30 formed therein which, in the stacked assembly, align to form fluid manifolds for supply and exhaust of reactants and products and, if cooling spaces are provided, for a cooling medium. Again, resilient gaskets or seals are typically provided between the faces of the MEA 5 and each of the cell separator plates 11, 12 around the perimeter of these fluid manifold openings 30 to prevent leakage and intermixing of fluid streams in the operating stack 100.